1. Field of the Invention
The present invention relates to a corona electrifying type electrification device which, is mainly used in an image forming apparatus, such as a copy machine, a printer and the like, and to a discharge wire used in the electrification device and to a method of manufacturing the discharge wire.
2. Description of the Related Art
Conventionally, electrification devices making use of a corona discharge phenomenon are widely used in image forming apparatuses, such as electrophotographic type copy machines, laser beam printers, and the like. Typical examples of such apparatuses are, for example, a primary electrification device for uniformly charging the surface of an image carrier on which an electrostatic latent image is to be formed, a transfer electrification device for transferring a toner image formed on the surface of the image carrier onto a member to be transferred, and the like.
FIG. 6 shows an example of a conventional electrification device. This type of the electrification device comprises a discharge wire 101 having a diameter of about 50 to 200 μm, an image carrier, a shield plate 102 as a confronting electrode disposed so as to surround the discharge wire with a portion thereof facing a member to be charged such as a member to be transferred, electrification blocks 103a and 103b in which the discharge wire 101 is stretched, and a high voltage power supply (not shown) for imposing a voltage capable of causing corona discharge from the discharge wire 101.
However, this type of corona type electrification device has a problem that when the discharge wire discharges, corona air currents are generated, and the discharge wire collects dust and the like contained in the air in the periphery of the electrification device and is polluted thereby.
In particular, in an image forming apparatus using an electrophotographic system, the discharge wires of a primary electrification device, a transfer electrification device and a separation electrification device collect toner floating in the peripheries of the discharge wires and are liable to be polluted. Moreover, the pollutants deposited on the discharge wires are baked on surfaces thereof by the discharge of the discharge wires and very strongly adhered thereon.
On the other hand, the primary electrification device, the transfer electrification device, the separation electrification device, and the like used in the image forming apparatus must uniformly discharge in the direction in which the discharge wires are stretched. However, it cannot be expected that the discharge wires polluted with toner and the like uniformly discharge, and, as a result, there is a problem that a good image cannot be obtained unless the discharge wires are frequently cleaned or replaced.
Specifically, if the primary electrification device and the transfer electrification device cannot uniformly discharge, the density of an image is made uneven. Further, when the separation electrification device cannot uniformly discharge, insufficient separation and retransfer are caused.
As a conventional art for solving the problems described above, there is proposed a system which includes a discharge wire cleaning member having a polishing force, which is sufficient to remove pollutants, such as toner and the like, strongly adhered on the surface of a discharge wire as disclosed in Japanese Patent No. 2,675,837.
There have been tried various kinds of discharge wires to withstand the discharge wire cleaning member having the strong polishing force. For example, in the combination of a tungsten wire as a discharge wire whose surface is plated with gold and a cleaning member having a polishing force capable of removing the pollutants deposited on the surface of the discharge wire, the cleaning member scrapes off even the gold plating on the surface of the discharge wire.
A plating having a thickness of at least about 0.3 μm is necessary to apply the gold plating uniformly. The chips of the gold plating, which are scraped off by the cleaning member are made to whisker-like chips whose size is as large as 0.1 to 2 mm in cooperation with the ductility of the gold and prevent the uniform discharge of the electrification device by themselves.
There is a system in which a tungsten wire as a discharge wire, whose surface is mirror-finished by electrolytic grinding (herein, the mirror finished tungsten wire is called a white tungsten wire), is combined with a cleaning member having a strong polishing force as another conventional discharge wire.
However, when the white tungsten wire is left as it is in an environment of high temperature and high humidity, the surface thereof is naturally oxidized. Further, since the state of the oxidation lacks uniformity, the uneven naturally-oxidized-state also prevents uniform discharge.
There is also proposed a discharge wire, which is composed of a tungsten wire oxidized by a positive means such as heating or the like as still another conventional discharge wire as disclosed in Japanese Unexamined Patent Application Publication No. 48-74231 and Japanese Unexamined Patent Application Publication No. 8-305135. However, the tungsten wire having been oxidized at a high temperature has an advantage and a disadvantage as described below.
Since the oxidized layer on the surface of the tungsten wire having been oxidized at high temperature (at least 650° C.) is very uniform and hard, it can prevent natural oxidation and at the same time the oxidized layer on the surface cannot be easily scraped off even by a cleaning member having a strong polishing force.
However, it is difficult to apply the strong oxidation processing only to the very thin surface layer of the surface of the discharge wire whose diameter is about 200 μm at the largest as described above, and, as a result, the oxidized surface of the discharge wire has a thickness of several microns, whereby the discharge wire is liable to be mechanically damaged by bending and the like.
A discharge wire which is mechanically fragile greatly impairs workability in its replacement, and the like, which is, needless to say, a disadvantage. Particularly, in a discharge wire having a diameter of 100 μm or less, which is excellent in discharge efficiency, it is difficult that a practically usable strength is compatible with strong oxidizing processing.